Simple question I think, but you guys make the call.

Can the cooling wire (Y) be used to activate the furnace fan?

My neighbor has a Honeywell RTH7400 programmable thermostat used for natural gas heat with fan only (he has no AC). I've already figured out how to turn the fan on manually, and that's OK.

I, however, was wondering if the cooling contact on the t-stat can be used to turn the fan on automatically if I set the t-stat to be a Heat and AC system.

The t-stat is presently set to 'heat only with fan'. Only reason I ask is 'cause I don't want to ruin anything.

Thanks in advance. I appreciate your input.

  • What's the make and model of the furnace?
    – Tester101
    Commented Aug 10, 2016 at 10:54
  • It's an Airtemp single stage 90% efficient upflow gas furnace...not sure of the model. Suffice it to say that my neighbor said it was 'cheap', and that's why he bought it. It's nothing special as far as I can tell.
    – Ferd
    Commented Aug 10, 2016 at 16:43

2 Answers 2


When the t-stat is set to cool the fan comes on automatically when the temperature is set below the house current temp. But the fan will not turn off unless the house temp falls below the set temp. The (y) terminal does not need any wire attached to it. But do not try to make any terminal connections for anything but what it is intended for. You can easily burn the transformer, fuses or the low voltage wiring!!!

  • When I say (y) terminal needs not be connected I mean from furnace block to outside AC unit you do need the (y) terminal on t-stat to the (y) terminal on the terminal block at the furnace to be connected.
    – yourby
    Commented Aug 9, 2016 at 19:22
  • @yourby...thank you! I believe you understand what I'm tryin' to say. So if I set the t-stat for heat AND cool (even tho there's no AC), the fan should run (if set to Auto) according to room temperature? I'm just kinda fishin' for ideas, if you get my drift. I am by no means a HVAC person, although I used to be an electrician, so I'm not totally in the dark... And as for making connections that I have no clue about, that would be the premise of my question...
    – Ferd
    Commented Aug 9, 2016 at 22:28
  • So my best guess is that if I change the setting on the t-stat to make it think there's AC, that the fan would turn on automatically when the temp is set below a certain degree...? Or am I totallly missing something?
    – Ferd
    Commented Aug 10, 2016 at 16:48

Correct me if I'm wrong. But the typical forced air heat/AC system has 3 control lines (not counting R or C)...

  • heat call, commonly W
  • fan call, commonly G
  • A/C compressor run call, commonly Y

If it needs heat it calls for heat and fan. If it needs A/C it calls for fan and A/C. I'm also taken to understand that many of these smart thermostats can sense whether apparatus is connected to a wire, and configure themselves accordingly.

So you want it to call for A/C, but since you don't have A/C, you want it to make a fan call instead. It already does that as part of calling for A/C.

The problem is that the system never calls for A/C, because it doesn't think you have an A/C system at all. You can try telling the system manually that you do have an A/C system. Many systems like the Nest won't believe you, they can detect that there's no A/C relay connected to the A/C call wire. I would connect a dummy load to that wire. Either a right-sized resistor in a proper, safe housing... or how about this -- an actual air conditioning relay! They are about $12 last I priced them. The relay does not care whether any power lines are wired to it. It would connect in the usual way between the A/C call wire and C. At this point the thermostat would make an A/C call and fan call in the normal way.

  • 1
    I think in the case of heat, most thermostats don't activate the fan, the fan is activated and deactivated by a fixed thermostat in the bonnet over the heat exchanger. Although that may only be true for forced air gas furnace.
    – Tyson
    Commented Aug 9, 2016 at 22:18
  • @Harper...NO, it does NOT already turn on the fan, but the furnace DOES have a Y terminal, so......
    – Ferd
    Commented Aug 9, 2016 at 22:31
  • @Tyson Most furnaces have a simple time delay for the fan, not a temperature sensor. But you're right, thermostats don't typically energize G during a call for heat.
    – Tester101
    Commented Aug 10, 2016 at 10:53
  • @Ferd i think it doesn't' because it never calls for A/C, because it does not believe an A/C unit is present. As Tyson and Tester101 are saying, it doesn't activate G either since the furnace does that on its own. So if the furnace allows you to hook up a G wire, I would hook The G wire there, and a dummy load on the Y, so the thermostat now thinks you have A/C. Alternately I suppose you could hook Y to G: The thermostat would be none the wiser that the A/C call is going to the fan, but it might notice the absence of anything on G. Commented Aug 10, 2016 at 14:00
  • @Harper...please pardon my last comment to you. I failed to read your post in its entirety. Makes better sense to me today.
    – Ferd
    Commented Aug 10, 2016 at 16:44

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